In recent years, fuel cells have been put into practical applications as electric power generation means accompanied by less CO2 emission. Although several types of fuel cells have been proposed, fuel cells which are generally referred to as PEFC (polymer electrolyte fuel cell), have been extensively developed as a typical type among them.
PEFC has a structure which includes a polymer electrolyte membrane sandwiched between electrodes (an anode and a cathode) supporting a catalyst such as Pt. By supplying a gas that contains hydrogen to the anode, and a gas that contains oxygen to the cathode, hydrogen is separated on the anode into a proton and an electron by an action of the catalyst.
Thus generated proton is transferred together with H2O in the form of an oxonium ion H3O+, and supplied to the polymer electrolyte membrane. In the polymer electrolyte membrane, only the protons migrate, the protons, oxygen and electrons form a so-called three-phase interface, on the catalyst of the cathode. Accordingly, a reaction that produces water from oxygen and the proton is caused, and thus electric power generation by the fuel cell is executed. Namely, it is important for electric power generation by fuel cells that only the protons migrate in the polymer electrolyte membrane, and that hydrogen, oxygen, and electrons are not transmitted to the opposite electrode.
Thus, in fuel cells typified by PEFC, in which electric power generation is executed by proton transfer, proton conductor plays an important role in proton transfer at the electrode and the electrolyte membrane. Therefore, proton conductors have been extensively developed, along with the development of fuel cells.
However, the electric power generation efficiency of PEFC is still so low. Therefore, improvement of the reaction efficiency of the catalyst in the electrode, and improvement of the proton conduction in the polymer electrolyte membrane are important for improving the electric power generation efficiency of PEFCs. In particular, since the reaction efficiency of the catalyst greatly depends on the reaction temperature, the most effective means for increasing electric power generation efficiency is elevation of the reaction temperature of the catalyst of PEFC, namely elevation of the operation temperature of PEFC.
However, since mechanisms of proton transfer in the polymer electrolyte membranes involve oxonium (H3O+) conduction via H2O that is present in the polymer chain, it is difficult to allow PEFC to generate electric power under an ambient pressure at not lower than 100° C. This is because water vaporizes under an ambient pressure at 100° C., and it is difficult to permit stable presence of H2O in the polymer electrolyte membrane at not lower than 100° C.
On the other hand, some of phosphoric acid type fuel cells (PAFCs) have been put into practical applications. Since the PAFCs use phosphoric acid as an electrolyte, electric power generation at around 200° C. is enabled. However, PAFC involves a specific problem of necessity for measures against corrosion and leakage of liquids since phosphoric acid that is an acidic liquid is used.
Accordingly, operation of fuel cells within a temperature range of approximately 100° C. to 300° C., which is referred to as middle temperature range, is advantageous. However, there still exists a problem of absence of a proton conductor which is most suited for such fuel cells.
Proton conductors are broadly classified into proton conductors intermediated by H2O (i.e., proton conductors in which conduction of H3O+ is utilized), and proton conductors that conduct H+ without intermediation of water. According to the proton conductors intermediated by H2O, it is difficult to allow H2O that serves as a conducting medium to be stably present at a temperature within the range of not lower than 100° C. On the other hand, as the proton conductor that conducts only H+, perovskite type oxides such as SrZr1-xYxO3 based conductors have been known. However, in the case of the proton conductors composed of perovskite type oxides, the temperature at which proton can be conducted is not lower than 400° C.; therefore, operation temperature is likely to be too high.
As described above, there hardly exist proton conductors without intermediating H2O which are stably present within a temperature range of not higher than 300° C. Particularly, for realizing fuel cells that are operable in a temperature range of 100° C. to 200° C., the most crucial point is as to how stably H2O is allowed to exist in a proton conductor.
Transmission of fuels (H2, O2) in the fuel cells may lead to deterioration of electric power generation efficiency. Therefore, proton conductors desirably have low gas permeability such as, for example, compact membranes and gels. Similarly, also in light of affinity and adhesiveness with an electrode, membranes or gels being compact are desired.
Conventionally, a variety of materials that exhibit proton conductivity at a temperature of not lower than 100° C. have been proposed.
For example, according to Nonpatent Document 1, tin oxide hydrates (SnO2.nH2O) have favorable proton conductivity. SnCl4 that is a water soluble substance is dissolved in water, and then an equal amount of NH4OH is added thereto to allow Sn(OH)4 to be formed. Thus obtained precipitates, Sn(OH)4 are sufficiently washed with water to remove NH4+ and Cl− ions, and then heated to 110° C. to obtain SnO2.nH2O powders.
In Nonpatent Document 1, SnCl4 is used as a water soluble Sn compound and neutralized with NH4OH to form Sn(OH)4, which is a hydrozide salt. It is to be noted that Sn(OH)4 and SnO2.nH2O are insoluble in water.
The SnO2.nH2O powders include both adsorbed water and bound water. Nonpatent Document 1 reports that when weight variation from room temperature to 500° C. is studied, the adsorbed water desorbs in a temperature range of 25° C. to 130° C., and then the bound water desorbs in a temperature range of 150° C. to 350° C. On the basis of the weight when the temperature is elevated to 350° C., desorption of water of not less than 50% occurs through elevating the temperature by 200° C.
SnO2.nH2O disclosed in Nonpatent Document 1 is powdery. When adsorbed water more likely to be desorbed is removed, the amount of H2O relative to the entire powder is approximately 20% by weight in the case of n=2.
Patent Document 1 has proposed tin oxide hydrate, namely a complex of SnO2.nH2O obtained by conjugation with ceramic porous particles in order to maintain a water holding capacity in a temperature range of not lower than 100° C. Patent Document 1 discloses that ceramics porous particles are dispersed in, for example, a SnO2.nH2O matrix to provide the hydrate with moisture supplied from pores of ceramics particles, and thus water holding capacity is improved as compared with the case of SnO2.nH2O alone even in a temperature range of not lower than 100° C.
Therefore, the conductors disclosed in Patent Document 1 are powders and molded products thereof, and the proportion of H2O adsorbed in the porous particles relative to the H2O content similar to that of Nonpatent Document 1 will account for the proportion of the adsorbed H2O amount relative to the entire H2O.
Patent Document 2 discloses a solid electrolyte membrane that is a layered phosphoric acid compound (Sn(HPO4)2.nH2O etc.), which is characterized by having a molar ratio of Sn/P of 0.3 to 0.8 . Patent Document 2 discloses that crystalline tin phosphate results in a high proton conductivity of approximately 10−2 to 10−3 S/cm at 150° C.
According to Patent Document 2, the layered phosphoric acid compound is powdery.
The Nonpatent Document 2 discloses that a mixture of nanoparticles SnO2 and phosphoric acid (H3PO4) is superior in thermal stability. Furthermore, Nonpatent Document 2 also discloses that mixing of nanoparticles SnO2 with phosphoric acid improves water holding capacity. Namely, network between SnO2 and H3PO4 is disclosed to provide high water holding capacity.
Patent Document 3 discloses a proton conductor having a P2O5 or phosphoric acid group on the fine pore surface or inside the fine pore structure of a nano porous material of one metal oxide selected from tin oxide, vanadium oxide, tungsten oxide and manganese oxide. According to Patent Document 3, by forming P2O5 or phosphoric acid groups in fine pores, the P2O5 or phosphoric acid groups reportedly bind to water by a binding force that is greater than van der Waals attraction.
The proton conductor disclosed in Patent Document 3 is powdery. Thus, in order to improve water holding capacity, doping of nano porous fine pores with P2O5 or phosphoric acid groups is carried out. Construction of a network consisting of fine pores of oxides, P2O5 or phosphoric acid groups, and H2O improves the water holding capacity. Patent Document 3 describes that the network disclosed in Patent Document 3 is superior in water holding capacity as compared with the SnO2—H3PO4 network disclosed in Nonpatent Document 2.
As gelatinous substances in which SnO2 is used, Patent Document 4 discloses a gel in which SnO2, Sb, and NH3 are used. According to Patent Document 4, the gel is produced in the step of preparing a material having electron conductivity of SnO2.
As in the foregoing, a large number of proton conductors for operating fuel cells at temperatures of not lower than 100° C. have been proposed.
As an example of a practically applied fuel cell operated in a temperature range of approximately 200° C., PAFC in which liquid H3PO4 is used has already put into practical applications (Nonpatent Document 3).
[Citation List]
[Patent Documents]
Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-273286
Patent Document 2: Japanese Patent Laid-Open Publication No. 2005-285426
Patent Document 3: Japanese Patent Laid-Open Publication No. 2007-66668
Patent Document 4: Japanese Patent Laid-Open Publication No. 1-257129
Patent Document 5: Japanese Patent Laid-Open Publication No. 2008-243688 (in particular, paragraph No. 0034)
Patent Document 6: Japanese Patent Laid-Open Publication No. 2008-020411 (in particular, paragraph Nos. 0013 to 0017)
[nonpatent documents]
Nonpatent Document 1: Shinji Hara et. al. Solid state Ionics 154-155 (2002) 679-685.
Nonpatent Document 2: Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 147-154.
Nonpatent Document 3: Fuji Jihou Vol. 75 No. 5 (2002) 285.